Washington's chief nuclear negotiator in six power talks with Iran says the week of meetings that ended in Vienna on June 20 was "very tough but constructive."

But Wendy Sherman added, 'It is still unclear whether Iran is really ready to take all the steps necessary to assure the world that its nuclear program is and will remain exclusively peaceful."

EU spokesman Michael Mann told reporters that Iran and the six world powers will meet again in Vienna on July 2 and have begun the process of drafting a final agreement to end the crisis over Tehran's nuclear program.

But Iranian Foreign Minister Mohammad Javad Zarif said they have yet to find common ground on the main issues.

Zarif urged the six nations to "abandon excessive demands which will not be accepted by Iran."